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          <h1 class="post-title" itemprop="name headline">Java四种引用</h1>
        

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        <p>java和c++最典型的区别就是内存的管理方式不同，java的内存管理是通过虚拟机来实现的，通过垃圾回收器释放内存。c++通常情况下是通过程序员手动释放。为什么说是通常呢？在c++有智能指针，通过引用计数方式释放内存。</p>
<p>在c++11中，智能指针分为强引用指针、强指针引用和弱引用指针。强引用指针每个指针独享一块内存区域、强指针引用引用是大家共享一块堆内存，弱引用指针很多时候在配合强指针引用使用，动态的知道所指的对象是否活着。还有就是解决循环引用的问题。java的引用类型和c++的智能指针有一定相似的地方。</p>
<p>java中的引用类型分为强引用、弱引用、虚引用和软引用四种。</p>
<p>强引用</p>
<p>Object object=new Object();</p>
<p>这是java通常的创建对象的方式,jvm不会回收强引用所引用的对象，即使内存不足，发生OOM错误。</p>
<p>如果将object=null;那么在下次触发gc的时候我们new出来的这个对象就会被销毁。</p>
<p>弱引用</p>
<p>无论内存是否充足，在进行垃圾回收的时候，jvm都会将弱引用所引用的对象回收</p>
<p>String str=new String(“hello world”);<br>WeakReference<string> sf = new WeakReference<string>(str);<br>str=null;<br>System.out.println(sf.get());<br>System.gc();<br>System.out.println(sf.get());<br>首先会输出hello world,然后输出null。在进行垃圾回收的时候，若引用关联的对象会被回收。<br>软引用<br>如果一个对象被软引用所引用，那么在内存不足的情况下，会被jvm回收。</string></string></p>
<p> String str=new String(“hello world”);<br>       SoftReference<string>sr=new SoftReference<string>(str);<br>       List<object>data=new ArrayList&lt;&gt;();<br>       while (true){<br>           System.out.println(sr.get());<br>           data.add(new String[1024]);<br>       }<br>这段程序刚开始输出的是hello world,一段时间后输出null，最后发生OOM错误退出。jvm参数设置不同，效果有所区别。当内存不够时，jvm会对软引用进行回收。</object></string></string></p>
<p>虚引用</p>
<p>如果一个引用和虚引用关联，则和没有引用关联一样，jvm会将其回收。虚引用需要和引用队列联合使用，需要将虚引用加入引用队列中，如果虚引用已加入引用队列，那么会被虚引用引用的对象将会被回收。<br>ReferenceQueue<string>queue=new ReferenceQueue&lt;&gt;();<br>String str=new String(“hello world”);<br>PhantomReference<string>ph=new PhantomReference&lt;&gt;(str,queue);<br>System.out.println(ph.get());</string></string></p>
<p>这里会输出null。</p>

      
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